The present invention concerns a process for manufacturing an electrically insulating and mechanically structuring sheath on an electric conductor.
The invention applies notably to the manufacture of superconducting magnets as well as the manufacture of pole pieces for an electric motor.
The need to have a structuring electrical insulation, obtained from a flexible ceramic precursor, is common to numerous electrotechnical applications.
The precursor is shown, according to an embodiment of the invention, in the form of a supple impregnated fabric, which facilitates the shaping of the conductor that is to be insulated and, in certain cases, ensures a check on thickness and determines the geometrical precision of windings realized by the conductor.
After a heat treatment at a temperature in the region of 700xc2x0 C. the precursor is sintered and the electrical insulation thus constituted absorbs the mechanical forces exercised on the conductor at the time of subsequent assembly and operating phases. The ceramic type of insulation realized accepts operating temperatures from 1.8 K to 1270 K.
These properties are essential at the moment of realizing superconducting magnets, capable of generating intense magnetic fields where a major difficulty is encountered.
Indeed, the materials with superconducting properties and the capacity to make high current density circulate, including the intermetallic alloy defined Nb3Sn or even Nb3Al require a heat treatment at high temperature (above 600xc2x0 C. for Nb3Sn and 700xc2x0 C. for Nb3Al), of long duration (above 100 hours for Nb3Sn and in the region of some dozen hours for Nb3Al) and in an inert atmosphere or under vacuum.
The temperature conditions mentioned above prohibit the use of conventional electrical insulators realized on the basis of organic products, during heat treatment. What is more, the superconducting material obtained after this heat treatment is fragile and the possible mechanical stresses which are liable to be applied on it can easily downgrade or harm its superconducting properties.
It is therefore no longer possible, after the heat treatment, to ensure the shaping of this material, nor the bending required for its winding. In these conditions, the fitting of the electrical insulation is particularly awkward.
The conventional solution to remedy the above disadvantages consists in:
taping a superconducting cable, before its winding, using a mineral fibre tape accepting the heat treatment,
carrying out this heat treatment, then
placing the winding in an impregnation mould under vacuum, and
applying an organic resin impregnation.
The operations of transfer of a winding are particularly awkward and have, to this day, never allowed series production of complex windings (of the dipole or quadrupole type), of considerable size (notably of more than 1 meter), using superconductors of the Nb3Sn family.
The cost of the superconducting materials used (in the order of 750 C/Kg to 2000 C/Kg depending on the realization processes used), as well as treatment time and the length of winding operations represent more than 30% of the cost of manufacturing superconducting electromagnets.
The risk relating to the transfer of the reaction mould (in which the superconducting precursor is transformed into superconductor) to the impregnation mould is therefore very important.
The possibility of having a completely insulated winding with a complete mechanical integrity after the reaction treatment of the superconductor would make it possible to develop industrialization of the superconducting electromagnets.
Electrical insulation techniques are already known on superconducting electromagnets in Nb3Sn. But all these known techniques need an impregnation using epoxy resin and do not provide the mechanical resistance of the superconducting electromagnet winding to resist the magnetic forces generated by the workings of the electromagnet in intense fields.
Other known techniques use a ceramic insulation.
This is particularly referred to in the following document:
EP-A-0044144 (invention of G. R. Sutcliffe, S. J. Warden and D. Humpherson) corresponding to U.S. Pat. No. 4,407,062.
However, all these other recognized techniques consist in depositing an insulator around the strands of a superconducting material, either by passing these strands through a solution of an inorganic precursor, or by extrusion of the precursor around strands through dies, and none of these other known techniques permits having a mineral fibre ribbon, this ribbon being pre-impregnated with the precursor of a ceramic matrix.
The aim of the present invention is to resolve the disadvantages of the known techniques for the manufacture of electrically insulating sheaths on electric conductors in particular those made in superconducting material.
The object of the invention is a manufacturing process for electrical insulator which can be laid on a conducting wire or with which this conductor can be taped, in particular in the case of a conductor designed to be coiled, the process which allows taping the conductor with the insulator or laying the latter, before the winding of the conductor.
This process aims also at giving a certain flexibility to the conductor thus coated, this flexibility facilitating wrapping and especially winding this conductor.
What is more, this process makes the synthesis of a ceramic material possible at the time of a heat treatment.
In the particular case of winding, the invention gives the following results, notably in the case of a superconducting conductor:
the electrical insulation of the conductor is adequate,
the mechanical cohesion of the winding at ambient temperature is acceptable,
this mechanical cohesion is maintained at the time of cooling the insulated conductor by liquid helium as well as supply of the winding in current,
control over winding dimensions is acceptable, in particular as far as the spacing of the winding turns is concerned, and this is the case at any temperature, and
the winding conveniently has a certain porosity to liquid helium.
What is more, the ceramic insulator manufactured in compliance with the invention is free of organic phase after the heat treatment and does not require the addition of an organic phase to attain its properties of electrical insulation.
Moreover, in a particular realization mode, this insulator is formed of a reinforced ceramic matrix of short ceramic fibres.
In a precise manner, the object of the present invention is a manufacturing process for an electrically insulating and mechanically structuring sheath on an electric conductor, in particular a conductor in non-superconducting metal or a conductor in superconducting precursor, this process being typified in that it comprises the following stages:
formation of a ceramic precursor in gel form,
formation of a conducting coating with this ceramic precursor in gel form and thus without deposition, and
heat treatment of this coating, this heat treatment being suitable for forming the ceramics from the ceramic precursor in gel form.
According to a preferred implementation mode of the process of the invention, the ceramic precursor is a liquid constituted by a solution comprising water, a mineral component, chosen from among boehmite and clays from the kaolin family, and an organic binding agent, and the mineral component is made to react with an acid to gel the solution and thus obtain the ceramic precursor in gel form.
The acid can be chosen from a group comprising boric acid, citric acid, hydrochloric acid, nitric acid and the carboxylic acids, preferentially formic acid.
The solution, besides, can comprise glass frit and/or at least a supplementary mineral oxide.
According to a particular realization mode of the invention, the solution comprises, in weight percentage, 35% to 45% water, 8% to 30% mineral component, 1% to 10% organic binding agent, 0% to 15% of a single or plurality of supplementary mineral oxides and a complement of glass frit, this complement of glass frit if any ranging, of course, from 0% to 56%.
It should be noted that the role of the water in this solution is to make the mix fluid and also to hydrate the mineral oxides, notably the boehmite and kaolin. In the subsequent treatment, the water is evaporated in the course of a drying operation and, consequently, it would be totally possible to increase slightly artificially the percentage of water in the solution and to reduce proportionately the percentage of the other constituents, without leaving the context of the invention, for all that is needed would be, at this stage, to prolong the drying phase to eliminate the surplus water.
Obviously one should avoid adding too much water which would deteriorate the gel.
According to a preferred realization mode of the invention, the weight percentage of the mineral component is between 15% and 30% in this solution, the complement of glass frit, if any, is therefore of course between 0% and 49%.
Each supplementary mineral oxide can be chosen from the group comprising alumina, zirconia, silica and the alumino-silicate clays.
According to a preferred realization mode of the process of the invention, this process comprises furthermore an elimination stage of the organic binding agent after the formation stage of the coating, this elimination stage beginning before the suitable heat treatment stage to shape the ceramic but ending during this heat treatment, in such a way that these two stages partially overlap over a period of time.
Preferably, the process comprises furthermore an elimination stage of the organic binding agent through reaction with the oxygen, after the formation stage of the coating.
According to a specific first mode for implementation of the process of the invention, the conductor is in precursor of the superconductor Nb3Sn or Nb3Al, and an overall heat treatment of this conductor provided with the coating is carried out, this overall heat treatment being carried out in a neutral atmosphere and suitable for forming the superconductor Nb3Sn or Nb3Al, eliminating the organic binding agent and forming the ceramics.
According to a specific second mode for implementation, the conductor is in precursor of a copper oxide based superconductor in particular YBa2Cu3O7, Bi2Sr2CaCu2O2 or Bi2Sr2Ca2Cu3O10, and the organic binding agent is eliminated through heating, in the air, of the conductor provided with the coating, then an overall heat treatment is carried out in the air of the conductor provided with the coating, this overall heat treatment being suitable for forming the copper oxide based superconductor and forming the ceramics.
According to a specific third mode of realization, the conductor is in a non-superconducting metal and an overall heat treatment is carried out on this conductor provided with the coating, this overall heat treatment being carried out in a neutral atmosphere and suitable for eliminating the organic binding agent and forming the ceramics.
The formation stage of the coating can comprise a stage of deposit of the ceramic precursor in gel form on a ceramic fibre fabric previously de-sized (that is to say a ceramic fibre fabric where the sizing has previously been removed), hen an arrangement mode of the fabric provided with the ceramic precursor around the conductor.
The ceramic fibres can be made in a material chosen from E glass, C glass, R glass, S2 glass, Silica, Alumina and Mullite.
The ceramic fibre fabric can be previously de-sized by heat or chemically.
According to a specific realization mode of the invention the conductor provided with the coating is put into shape and in particular coiled, before the heat treatment stage suitable for forming the ceramics.